164 CHEMICAL AGENTS AND GROWTH 



the basal medium. When the discs were exposed to red light, they 

 attained about a 50% increase in area, whereas those grown in dark- 

 ness attained a 20 to 25% increase. Under ideal conditions the discs 

 exposed to far red and red-far red grew approximately the same as 

 those in darkness, although they often grew slightly more, indicating a 

 partial activation of the red-sensitive pigment system. Controls in 

 basal medium were run for each experiment in order to estimate the 

 variability in growth between experiments. 



Before extensive experimentation was begun, it was felt that we 

 should examine our system for its similarities to the lettuce seed sys- 

 tem. We examined the system for infinite reversibiUty by red and far 

 red and also studied the effect on growth of a time lag between the 

 exposure to red and to far red. A series of petri dishes containing 20 

 discs each was placed in the darkroom. One dish was kept in con- 

 tinuous dark as a control. Another dish was exposed to far-red light 

 and returned to darkness. All remaining dishes were exposed to red 

 light simultaneously, a single dish then being removed to darkness; 

 the remainder were then exposed to far-red light. A sequence of such 

 red and far-red exposures was given, a single dish being removed after 

 each light exposure. The results given in Table I show that the 



Table I. Photoreversibility of Etiolated 

 Bean Leaf Disc Expansion 



Treatment Increase in Diameter, mm 



Dark 0.66 



Far-red light (I) 0.69 



Red light (R) L36 



RI 0.97 



RI-R L43 



RI-RI 0.93 



RI-RI-R 1.36 



RI-RI-RI 0.96 



RI-RI-RI-R 1.32 



RI-RI-RI-RI 0.95 



RI-RI-RI-RI-R 1.43 



response is dependent upon the quality of the light given during the last 

 exposure. In additional experiments wherein we exposed the discs to 

 red light and waited varying periods of time before exposure to far 

 red, we observed that the discs escape slowly from control by far red 



